Conventionally, in a radio communication system for transmitting and receiving packet signals, Hybrid Automatic Repeat Request (HARQ), which is a technology for improving an error rate of the packet signals, has been used (see Non-patent Document 1, for example) Even when detecting an error in a received packet signal, a communication device equipped with the HARQ function does not discard the packet signal from which the error has been detected, but combines the packet signal with a retransmitted packet signal.
A general configuration of the communication device equipped with the HARQ function will be described with reference to FIG. 1. A description of the communication device shown in FIG. 1 is given by taking as an example of a case where HARQ is performed by using a Chase Combining (CC) scheme. As shown in FIG. 1, the communication device equipped with the HARQ function includes an error detector 41, a memory 42, a retransmission request generator 43, and a combiner 44.
The error detector 41 performs a decoding operation, such as error correction, on a signal outputted from the combiner 44, and then detects whether or not the signal has an error. Furthermore, the error detector 41 supplies a control signal which is based on whether or not there is an error, to the memory 42 and the retransmission request generator 43.
In response to the control signal from the error detector 41, the memory 42 saves the combined packet signal. In the following, the packet signal saved in the memory 42 is referred to as a “saved signal,” as appropriate.
Upon detection of the necessity of a retransmission request in response to the control signal from the error detector 41, the retransmission request generator 43 transmits a retransmission request signal to a sender of the packet signal.
The combiner 44 calculates a combining weight of the retransmission packet signal (hereinafter referred to as a retransmission signal) and the saved signal, on the basis of a preset combining weight calculation scheme. Then, the combiner 44 generates a combined packet signal (hereinafter referred to as a combined signal) by combining the retransmission signal and the saved signal.
Specifically, the packet signal includes a known signal indicating information, such as a training sequence (TS), known by the receiving side, and an unknown signal indicating content of communication data not known by the receiving side.
Then, by using the preset combining weight calculation scheme such as Minimum Mean Squared Error (MMSE) scheme, the combiner 44 calculates a combining weight used to combine the saved signal and the retransmission signal, on the basis of the known signal included in the saved signal and the known signal included in the retransmission signal. Then, the combiner 44 combines the saved signal and the retransmission signal on the basis of the calculated combining weight.
In this way, the communication device equipped with the HARQ function calculates the combining weight on the basis of known signals included in the saved signal and the retransmission signal, and combines the saved signal and the retransmission signal on the basis of the calculated combining weight. This is performed to improve the error rate in the combined signal even when a received signal includes a noise signal such as a delay signal or an interference signal.
In a radio communication system in which radio communications are performed among multiple communication devices at the same timing and over the same channel by means of Space Division Multiple Access (SDMA) scheme or the like, a packet signal sometimes includes, as a large noise signal, an interference signal of a radio signal transmitted to another certain one of the communication devices.
FIGS. 2(a) and 2(b) show a relationship of packet signals on the time axis when communication devices 1 and 2 detect error in their respective packet signals received at the same timing t(m) because of their respective interruption signals, request retransmission, and receive retransmission signals at the same timing t(m+D).
In FIGS. 2(a) and 2(b), “m” represents a packet number, and “D” represents the number of packets before the retransmission signals. In addition, “TS1” and “TS2” represent known signals included in the packet signals received by the communication device 1 and the communication device 2, respectively. “Data1_m” and “Data2_m” represent unknown signals, such as data, included in the packet signals received by the communication device 1 and the communication device 2, respectively.
As shown in FIG. 2(a), the communication device 1 receives packet signals having the same packet number, at the timing t(m) and the timing t(m+D). Similarly, the communication device 2 receives packet signals having the same packet number, at the timing t(m) and the timing t(m+D). Thus, an interference signal included in the packet signal received by the communication device 1 at the timing t(m) is almost equivalent to an interference signal included in the packet signal received by the communication device 1 at the timing t(m+D).
When combining the packet signals with the HARQ function, the communication device 1 calculates a combining weight by referring to the known signal TS1 included in the saved signal received and saved at the timing t(m), and to the known signal TS1 included in the retransmission signal received at the timing t(m+D). Then, the communication device 1 combines the saved signal and the retransmission signal on the basis of the calculated combining weight, and thereby obtains a combined signal in which an interference signal in the unknown signal Data1_m including data or the like is decreased to improve the error rate.
[Non-patent Document 1] “Retransmission Control Scheme of Hybrid ARQ in MC-CDMA,” Ken'ichi Miyoshi, and four others, Journal of Institute of Electronics, Information and Communication Engineers, B, Vol. J89-B, No. 2 pp. 182 to 194